This relates to a method and apparatus for X-ray fluorescence (XRF) spectroscopy. The techniques and equipment described are particularly useful for a quantitative borehole analysis of the elemental content of surrounding rock. Numerous other applications will also be apparent.
Each chemical element has a distinctive X-ray line spectrum having energies and therefore wavelengths that are dependent only upon the atomic number Z of the element. As a result it is possible to identify the presence of the element from observation of one or more of its distinctive line spectra. These distinctive X-ray lines are produced when an electron from one of the outer shells of the atom takes the place of an electron that was previously removed from an inner shell. The K spectra arise from electron transitions from the L to K shell which give rise to the doublet K.alpha..sub.1 and K.alpha..sub.2 and M to K transitions which produce K.beta..sub.1 and K.beta..sub.2. The L spectra have a dozen or more lines of longer wavelengths produced by transitions from the M and upper shells to the L shell. The higher atomic number elements also have M and N spectra.
To remove an electron from an inner shell, it is necessary to bombard an atom with a high-energy electron beam or with a high energy beam of electromagnetic radiation such as X-rays or gamma rays. The energy required to cause such electron vacancy must exceed the binding energy of the electron in its shell.
In addition to the line spectrum, a continuous, nearly structureless, background spectrum of wavelengths is generated by such bombardment. When an X-ray tube is used to generate a stream of high energy bombardment electrons, this spectrum is produced by the rapid deceleration of electrons in the target of the X-ray tube. Where high energy electromagnetic radiation is used, the continous spectrum is generated by the collisions of photons of electromagnetic radiation with electrons in accordance with the well known Compton effect. Considerable additional information on XRF analysis may be found in R. O. Muller, Spectrochemical Analysis by X-ray Fluorescence (Plenum 1972).
The intensity of the radiation in any XRF spectral line is a measure of the concentration of the element which produced such spectral line. As a result, the techniques of XRF analysis are widely used for analyzing materials such as ores, soils, glasses, catalysts, alloys, clays, dusts, paints, silicates, and the like to determine their elemental composition and concentration. For example, the techniques can be used for trace analysis in a laboratory or for on-stream analysis of process streams.
One application of particular interest to the present invention is the use of XRF analysis in mineral assays. Until recently, such use of XRF analysis was limited to the laboratory. A sample ore to be examined was obtained from the mine, prepared for analysis in the form of a solution or fine powder and irradiated with a source of high energy electrons, X-rays or gamma rays under controlled laboratory conditions. The resulting X-ray spectrum was then measured over the wavelengths of interest to determine what minerals were present and in what concentration. Obviously, however, this process is time consuming and does not permit on-site assays.
In U.S. Pat. No. 4,045,676, which is incorporated herein by reference, a technique is described which permits the use of XRF analysis at the rough surface of a mine face. In this technique a reference sample is prepared having a known concentration of the element which is to be assayed. The reference sample is then irradiated by a suitable source and a broad spectrum of background radiation from the sample is measured by the detector. For the same geometrical relation of source, detector and sample, a measurement is also made of the intensity of X-ray radiation having the characteristic energies and therefore wavelengths of the element whose concentration is to be determined. To assay this element in the matrix of rock at the mine face, the mine face is irradiated by the same source and the distance between the source/detector arrangement and the mine face is changed until the intensity of background radiation measured by the detector is the same as that from the reference sample. A measurement is then made of the intensity of X-ray radiation having the characteristic energies of the element being assayed. From this measurement and from the previously established relation between spectral line intensity and background in the sample of known concentration, an estimate of the concentration of the element is made.
This technique, however, requires the use of at least one reference sample and the ability to control the physical relationship between the source/detector and the object to be examined. While such control may be possible in the laboratory or at a mine face, it cannot be used in numerous other applications where assays are desirable. In addition, calibration of such device is a time consuming task which must be performed manually during each assay.